Synthetic polypeptides of alpha-amino acids as restrainers in photographic emulsions



Jan. 11, 1955 F w L E 2,699,391.

SYNTHETIC POL YPEFTIDES 0F d-AMINO ACIDS AS RESTRAINERS IN PHOTOGRAPHIC EMULSION-S Filed Jan. 11, 1950 Fig. Fig. 2

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LOG EXPOSURE LOG EXPOSURE f Fig. 7

0,. E Q Fr/fz W H Mueller IN V EN TOR.

L06 EXPOSURE -7 United States Patent SYNTHETIC POLYPEPTIDES OF tat-AMINO ACIDS 31% 1ESESTRAINERS IN PHOTOGRAPHIC EMUL- Fritz W. H. Mueller, Binghamton, N. Y., assignor to General Aniline & Film Corporation, New York, N. Y., a corporation of Delaware 1 1 This invention relates to photographic restrainers and particularly to synthetic polypeptides of a-amino acids as restrainers in photographic silver halide emulsions.

It is known in the art that different lots of photographic gelatin, even though made by the same manufacturer and under the same conditions, will vary considerably in photographic quality. For this reason, it is often desirable for the photographic, film manufacturer to start with a photographically inert gelatin and add to it the necessary active components, such as sensitizers, restrainers, gradation bodies, and stabilizers in known and calculated amounts. It is also desirable and of considerable advantage to be able to add compounds of well defined chemical composition and structure. While many compounds of defined structure are known which are efficient sensitizers and stabilizers, this is not the case with restrainers.

Restrainers, sometimes called anti-sensitizers, are known to occur in varying amounts in many gelatin types and perform a useful role in photographic emulsions by making extended ripening of the emulsion possible. This, of course, permits the attainment of maximum sensitivity while at the same time retarding the formation of fog nuclei to an extent which varies with diiferent batches of gelatin. The chemical composition of these natural restrainers is not known, nor is the exact mechanism of their action known, although it is thought to involve their adsorption to the silver halide crystal. However, one property of a restrainer appears to be the capacity to restrain the growth of the silver halide crystals in melted emulsions. This action is made manifest by a reduced sensitivity to light if the emulsion is ripened in the presence of a restrainer, but in the absence of sulfur sensitizers, when compared with the same batch ripened without the addition of a restrainer. Another very valuable property possessed by some but not all restrainers is the capacity to produce a steeper gradation in the emulsion during the ripening process.

One of the disadvantages of the restrainers occurring in commercial gelatins lies in the fact that their concentration and activity varies from batch to batch. These diflierences preclude the attainment of uniform photographic results unless the gelatins are painstakingly blended and evaluated by tedious empirical tests.

It is also known that certain substances of unknown structure are extractable from pure or impure proteins, such as glue or gelatin, by electro-dialysis. It has been reported that these substances, as well as decomposition products of vegetable or animal proteins, obtained by acid, alkaline, or fermentative hydrolysis with or without simultaneous partial oxidation, have been added to silver halide emulsions to increase their sensitivity. The

principal reason for adding these substances to silver halide emulsions was to take advantage of their sensitizing properties which are attributable in a large measure totheir labile sulfur content (C. E. K. Mees, The Theory of the Photographic Process, page 93, etc.).

Certain protein decomposition products of medium to high molecular weight, for example, lysalbinic acid, gelatose, and the like, have also been suggested by the art as additives to photographic developers so that after exposure and development softer silver images of finer grain may be obtained. These decomposition products may be obtained by the degradation of proteinous materials such as casein, gelatin, glue, or even leather wastes. Although these products yield softer silver images of finer grain produced by the development of an exposed 2,699,391 Patented Jan. 11, 1955 ICC silver halide emulsion, they do not act as useful restrainers when incorporated in silver halide emulsions in view of their apparent inability to repress the formation of fog nuclei.

In all of the foregoing cases, including hydrolysates of structural proteins and egg albumin-citrate combinations, respectively, the materials are of complex composition and contain, in addition to labile sulfur compounds, e. g., allyl isothiocyanate and allyl diethylthiourea, substances such as carbohydrates, purine derivatives, chondroitin, and the like, which are usually associated with natural proteins. In view of their uncontrollable labile sulfur content, the materials previously suggested or used may lead to excessive sensitization or to the formation of fog nuclei, especially during the ripening process, and in some cases, they cause other undesirable reactions in the emulsion, such as loss of contrast and the like.

It is an object of the present invention to provide synthetic polypeptides of a-amino acids as restrainers for silver halide emulsions, irrespective of the manner in which the latter are prepared, e. g., by heat treatment or by ammonia digestion.

Another object is to provide restrainers of a rather well defined composition which can be prepared free from contamination by undesirable labile sulfur compounds, and which exert a restraining action without imparting unwanted effects.

Still another object is to provide silver halide emulsions containing synthetic polypeptides prepared from pure amino acids or mixtures thereof.

Other objects and advantages will appear from the following specification.

The above objects are accomplished by the incorporation of synthetic polypeptides of tit-amino acids in silver halide emulsions. When so incorporated, the polypeptides exert a marked restraining action without imparting any adverse effect, such as loss of gradation or the formation of fog, the latter phenomenon being precluded by the absence of labile sulfur bodies which are responsible for the formation of these fog nuclei.

The synthetic polypeptides of wamino acids, somewhat related to proteins in their chemical structure but of well defined chemical composition and free of labile sulfur compounds, are obtained by the polymerization of N- carboxy-u-amino acid anhydrides or by the simultaneous polymerization or copolymerization of two or more different anhydrides of this type.

The synthetic polypeptides or polymers are prepared by allowing the anhydride of an N-carboxy-a-amino acid to react in a water-miscible alcohol, such as methanol, ethanol, propanol, and the like, containing some Water to catalyze the reaction. The formation of similar anhydrides and their intermolecular condensation products have been described by Leuchs (Ber. 39, 857- 61 [1906]) and more recently by Woodward and Schramm (J. Am. Chem. Soc. 69, 1551-2 [1947]).

For the purpose of the present invention, one may start with any one of the well known et-amino acids. The N-carbalkoxy and carbaralkoxy derivatives (carbethoxy), carbomethoxy, carbobenzyloxy, etc.) may be prepared by the usual methods given in the literature (Bergmann and Zervas, Ber. 65B, 1192 [1932] and Leuchs and Geiger, Ber. 41, 1721 [19081]), and the anhydrides may best be prepared by either of the two methods given in the following working examples.

While the N-carboxy-a-amino acid anhydrides themselves and/ or their polymerization products may be added to an emulsion as such, it is preferable from the manufacturing standpoint to allow the polymerization to proceed in a Water-miscible alcohol, to remove the alcoholinsoluble polymer, and to use the lower molecular weight polypeptides remaining in solution. The polymerization may also be carried out employing a mixture of two or more different N-carboxy-a-amino acid anhydrides, or the solutions of the polymers may be mixed either during or after polymerization. It is also possible to start out with a mixture of two or more amino acids, con vert them simultaneously into anhydrides, and finally sub ject the product to the polymerization reaction.

In general, the preparation of a simple N-carboxy-otamino acid anhydride and its conversion to a polypeptide can be best illustrated by the following equations utilizing alanine as the starting matenal:

Aqueous Nazc O Ha-GHC-O0H oi-oooonr N Hz Alanine (or-amino acid) Methyl chlorocarbonate (ehlorocarbonic ester) Polyalanyl alanine (polypeptide wherein X represents the number of alanine radicals contained in the polypeptide molecule.

In homogeneous as well as mixed polypeptides, prepared in accordance with the present invention and having the skeletal chain:

i- Jx the nitrogen and carbon substituents are determined by the a-amino acids and their derivatives employed as starting materials. .X represents the number of radicals contained .in the polypeptide molecule, and is believed to be an integer of relatively low value in accordance with the substantial water and alcohol solubility of the polypeptide. In view of the difliculty in assigning specific formulae for the various synthetic polypeptides of amino acids prepared .in accordance with the foregoing procedure and the following examples, they are clearly defined as synthetic polypeptides obtained by the polymerization of cyclic intramolecular anhydrides of N-carboxy-wamino acids and coploymers thereof in which a carboxy group and an N-carboxyamino group are joined to the same carbon atom.

The following are examples of a-amino acids and their derivatives which may be treated in accordance with'the. foregoing reaction to form 'the anhydrides: glycine, phenylglycine, alanine, phenylalanine, arginine, aspartic acid, 'y-monoesters of aspartic acid, citrulline, cystine, glutamic acid, fi-monoesters of glutamic acid, histidine, hydroxyglutamic acid, isoleucine, leucine, lysine, methionine, norleucine, proline, serine, threonine, tryptophane, tyrosine, valine, isovaline, and the like.

The polymerization products of N-carboxy-a-amino acid anhydrides, as well as copolymers thereof, may be utilized as restrainers in a restraining amount in any type of silver halide emulsion containing gelatin or other protective colloids, such as water-soluble cellulose derivatives, e. g., hydroxyethylcellulose, methycellulose, carboxyoxycellulose, partially hydrolyzed cellulose acetate; watersoluble salts of dicarboxylic acid esters of cellulose described in United States Patent 1,884,035; polyvinyl acetal resins; partially hydrolyzed polyvinyl acetate resins described in United States Patents 1,939,422 and 2,036,092; polyvinyl alcohol, modified polyvinyl alcohol, polyvinyl alcohol mixed with resorcinol, polyvinyl propionaldehyde and the like. They may be incorporated in the silver halide emulsion as ripening finals. By ripening finals is meant an addition during the ripening or "sensitivity-increasing stage of the emulsion making process. They may also be incorporated in the emulsion prior to ripening, for example, in the gelatin solution-utilized for the preparation of the emulsion, or in any other stage of the emulsion making prior to the final coating. The polymeric and copolymeric polypeptides are best used in concentrations of .002-2 grams per liter of wet emulsion.

The present invention is illustrated by the accompanying drawing in which Figures 1 to 7 represent typical characteristic curves of the silver-densities in sensitometrically exposed and processed silver halide emulsions prepared as described below by way of examples and in the manner set forth.

In Figures 1 to 6, wherein the ordinates represent densities and the abscissa represent the log of exposure, Type is the density curve for a sensitometrically exposed silver halide emulsion containing no restrainer and A, B, and C are the density curves for the same types of emulsions containing a 'restrainer and exposed under the same conditions.

The following examples will serve to illustrate certain ways in which the polymers and copolymers of the synthetic polypeptides are prepared and in which they are utilized as restrainers for silver halide emulsions, but are not to be construed as limiting the invention. All parts are by weight.

Example I Polypeptide of leucine,

To 10 parts of N-carbomethoxy-L-leucine in 25 parts of dry benzene, 6 parts of thionyl chloride were added. The mixture was warmed at EEO-40 C. for 1 hour, and the excess thionyl chloride was then distilled off under diminished pressure. The residue was heated at 70-80 C. for 1 hour at a pressure of 10-15 mm; of mercury. A tan-colored powder remained which is the crude N-car boxyanhydride. This product can be purified by .re crystallization from thoroughly dried ethyl acetate, or by dissolving it in dry ether and reprecipitating it with dry petroleum ether to give crystals melting at 7071 C. Either the crude or the purified material can be used in the subsequent steps: 7

A l per cent solution (weight/volume) .in per cent ethanol was prepared from the above crude product and allowed to stand until a solid had precipitated (about 3 hours). The solid was thenremoved and the clear solution containing the low molecular weight polypeptide was set aside. Two bromoiodide gelatin emulsions of normal speed and contrast, and containing the equivalent of grams of silver nitrate per liter, were prepared in the identical manner except that 10 cc. of the clear alcoholic solution prepared above (containing about 33 rngs. of the soluble material) were added to one of the emulsions before the precipitation. The two emulsions were coated on glass plates, dried, sensitometricallyexposed and proc essed under standard development conditions. The emulsion without the addition was identified as Type.

The density curves Type and A ,as illustrated in Figure 1, were obtained in the known manner. this figure, it will be observed-that curve A shows a definite restraining action with no loss in gradation as compared to the curve of a typeemulsion-containing no restrainer.

The magnitude of the restraining effect is distinctly noticeable regardless of what stage of the emulsion making or ripening the restraining substances are added.

Example ll Example I was repeated with the exception that 10 parts of N-carbomethoxy-L-leucine were replaced by'S parts of bis(N-carbomethoxy)-L-cystine. A l per cent solution in ethanol of the crude yellow-brownanhydride thus obtained was prepared. Ten cc. of the filtered solution were added to a liter of a gelatino silver'halide emulsion during its "precipitation. A typical characteristic curve from the sensitometric strip was obtained 'and'plotted as curve B in Figure 1. By reference to this curve, it will be noted that the presence of the restraining agent in a silver halide emulsion shows a good restraining action and some steepening of gradation. 7

Example II] t x Polypeptide of proline From Example I was repeated with the exception that .10 parts of N-carboinethoxy-L-leucine were replaced by 5 parts of N-carbomethoxy-L-proline. The crude brown anhydride was dissolved in alcohol to give a 1 per cent solution. 5 {S252 ig g zfi gp i ggfl g fi gg gg g g i g E i Z Zfi Q g g i i i gg. g isggg .2 gas. The unpurified product was dissolved in ethanol, I b S] n f S t ai filtered, and tested as in Example I. The compound an cuve F e sens} Ome no 5 HP was showed restralnmg action as can be seen from curve A as curve C in Figure 1. This curve shows restralning i Fi 3 action and improvement in gradation. 10 Example VIII ZHr-HOHa OHr-CHFCHz-Cflz-NHG O O CHzCuHs] H-HN-- HCO-HN H--CO OH L J: Ex m l 1V Copolyrnerization product (polypeptide) of leucine and lysine NH One part of L-leufiine-N-carboxyanhydride linger anhydride of a-(N-car oxyamino)-L-isocaproic aci pre- GHrCHTCHTTNHTgTNH pared as in Example I and 1 part of e-N-carbobenzyloxy- I L-lysine-a-N-carboxyanhydride [inner anhydride of t- ET H COIOH (N-carboxyamino)-e-(N-carbobenzyloxyamino) caprol: acid] prepared according to Example VII were mixe Polypeptide of arginine and dissolved in ethanol to give a 0.5 per cent solution. After standing for about 3 hours, the solution was filtered s? s???g ggf ggifiggfi i 5 3: iigg g g 2 and 12 of Ere filtate addeil beforti the precipitation I step in t e ma in 0 an emu sion. hotographic tests 3 22? gg g gfsg g fiz fig ig g i' g g gg l gz i carried out as in th e preceding examples showed the comtion. Ten cc. of the filtered solution (containing about Pound photographlc restramer as can be Seen from 37 mgs. or" the soluble material) were added to a silver curve B m Flgure halide emulsion during its ripening operation. A typical Example 1X H and D curve from the sensitometric strip was plotted r as curve A in Figure 2. This curve shows restraining An inert type of gelatin artificially deprived of its action and no loss in gradation. ripining and sensitizing constituents was dused in the ma ing of a photographic emulsion with an without the Example V addition of sulfur sensitizer. The results obtained by the l O 1 OH addition of 1 per cent solution of the lysine polymerizatron product of Example VII in 95% ethanol are shown Hz in liigure 4-. Curve A is the inert gelatin. Curve B 40 is inert gelatin contammg the lysme polymerizatlon product of Example VII, and curve C is inert gelatin CH containing the lysine polymerization product of Example x VII together with 4 cc. 05 1:1000 sodium thiosulfate solution as a sensitizer. T ese curves are plotted in com- Polypeptide of histidine parison to the curve from an emulsion made in the same Example I was repwted with the exception that 10 manner but using as type agelatin which was considered parts of N carbOmethoXy L 1eucine were replaced by 5 well balanced with respect to Its content of natural reparts of hl-carbomethoxy-L-histidine. The brown solid strailiing and Sansitizing bodies instead the inert was dissolved in alcohol to give a l per cent solution. gelatm' Ten cc. of tlljie filtered solution (containing about 77 mgs. Example X of the solu 1e material) were added. to a silver halide emulsion during its ripening operation. A typical H and g reapt$lypmiuct g W2 D curve from the sensitometric strip was plotted as curve a e e g k f i 6 B in Figure 2. This curve shows restraining action. z fi lz igg i gg i gf i g g gg g g'i g E l V1 eniulsign wits coated;1 sensitometrically exposettli, and deve ope n H an D curve made from t e sensito- CHPCHPO 00H metric strjif sll iowed giefinite restraicrliing action as is shown I" in curve lgure as compare to an emulsion made T QQT in an identical manner but containing none of the L- histidine polymer. Polypeptide of glutamic acid Exam le XI Example I was repeated with the exception that 10 p parts of N-carbomethoxy-L-leucine were replaced by 5 The Y Y P p from parts of N-carbomethoxy-D-glutamic acid. The viscous y Y- Y 38 F P VII Was converted IIILO oil was dissolved in alcohol to give a 1 per cent solution. a P Y E F y dlssolvlng 1H '4 Small 31110-1111! 05 hol Ten cc. of the filtered solution were added to a silver nd bollms f 3 h r T caused a p s precrpr halide emulsion during its ripening operation. A typical tatlOn 0f hep y Was dflegi 9 glVe a Whlte, H and D curve from the sensitometric strip was plotted tough, hom-llke materlal- Q P 1 OI t P y Was i a e o parso ogacla aceicaci an pars as curve C in Iugure 2. Th s curve shows restraining dd dt 25 t 'E 1 1 t d d 425 t action with no 1 in gradation 01f1 57 per centrhydrlctiillrt:i 3.1Cld.l Four parts 05 rid phosp orous were t en a e s ow y over a perio o 1 hour Example VII inhorder to remove the remaining e-carbobenzyloxy group. T e solid was removed by filtration, washed with cold CHTTOHTCHPCHTNHCoocmom water, and the washings combined with the filtrate. The water and acetic acid were then distilled under dimin- E EEC I ished pressure leaving a red oil. This oil was taken up Polypeptide of lysine derivative in 5 partsf ofswater arliad extractedhselverag tirraes with a;

.mixture 0 1 parts a so ute alco 0 an 20 parts 0 [Poly i carbobenzyloxylysme] dry ether. The material in the aqueous layer eventually wo arts 0 ot,ewar 0 enzy oxy- -ys1ne were 18- urne o a w to occu en preclpi a e w 1c was very T p f d'bb l l' d' t dt h'tfl lt '"tt hh solved in dry ether, the mixture cooled to 0 C., and 1 hygroscopic. Evaporatlon to dryness gave a colorless art of phosphorous pentachloride added. The mixture residue. A l per cent aqueous solution of this residue P was stirred for 20-30 minutes at 0 C., filtered with the was tested photographlcally m an emulsion as described exclusion of moisture and concentrated invacuo at 40-50 in Example I. The H and D curve showed restraining 61 C. Dry ethyl acetate was added and the concentrate distilled in vacuo at 50 C. The amber residue was taken up in dry ethyl acetate and precipitated with dry petroaction as can be seen from curve A in Figure 6 when compared to curve B which represents the photographic characteristics of an emulsion prepared in an identical manner but without the addition of the lysine derivative. The eifect of this material and sulfur sensitizers on an inert gelatin can be seen from Figure 7 wherein curve A represents an inert type gelatin; curve B represents the inert gelatin plus sodium thiosulfate; and curve C represents the inert gelatin plus sodium thiosulfate plus the polymer.

While I have herein disclosed the preferred embodiments of my invention, I do notdesire to limit myself solely to the specific examples, since it will be readily apparent to those skilled in-the art that precise ratios 'of the restraining agents may be varied depending upon the nature of the silver halide emulsion and the colloidal carrier without departing from the spirit and scope thereof. The precise ratios, i. e., restraining amount, can be very readily determined by simple trial experiments. .Accordingly, therefore, only such limitations should be imposed as are indicated in the appended claims.

I'claim: V

1. A photographic material comprising a base and a light sensitive silver halide emulsion containing, in a restraining amount, a polypeptide obtained by the polymerization of the cyclic intramolecular anhydrides of not more than two N-carboXy-a-amino acids in which a 'carboxyl group and an N-carboxy amino group are joined to the same carbon atom, said restraining amount being on the order of from 0.002 to 2 grams per liter of liquid emulsion used in the preparation of the emulsion layer.

2. A photographic material comprising a base and a light sensitive silver halide emulsion containing, in' a restraining amount, a polypeptide obtained by the homopolymerization of a single cyclic intramolecular anhydride of N-ca-rboxy-a-amino acid in which a carboxy group and an N-carboxyamino group are joined to the same carbon atom, said restraining amount being on the order of from 0.002 to 2 grams per liter of liquid emulsion used in the preparation of the emulsion layer. a

3. A photographic material according to claim 1 wherein said silver halide emulsion has gelatin as a carrier.

4. A photographic material according to claim 1 wherein said silver halide emulsion has a vinyl alcohol polymer as the carrier.

A photographic material comprising a base and a light sensitive silver halide emulsion containing, in a restraining amount, a polypeptide of low molecular weight and soluble in water miscible alcohols, obtained by the polymerization of the cyclic intramolecular anhydride of not more than two N-carboXy-a-amino acids in which a to the same carbon atom, said restraining amount being on the order of from 0.002 to 2 grams per liter of liquid emulsion used in the preparation of the emulsion layer.

6..A photographic material comprising a base and a light sensitive silver halide emulsion containing, in a restraining amount, a polypeptide of low molecular weight and soluble in water miscible alcohols, obtained by the homopolymerization of the cyclic intramolecular anhydride of a singie N-carboXy-a-amino acid in which a carboxyl group and an N-carboxyamino group are joined to the same carbon atoms, said restraining amount being on the order of from 0.002 to 2 grams per liter of liquid emulsion used in the preparation of the emulsion layer.

'7. A photographic material according to claim 5 wherein the silver halide emulsion has gelatin as the carrier.

8. A photographic material according to claim 5 where in the silver halide emulsion has a vinyl alcohol polymer as the carrier.

9. Aphotographic material according to claim 5 wherein a mixture of polypeptides as defined in said claim is employed.

'1-0. A photographic silver halide emulsion containing, in a restraining amount, a polypeptide of low molecular Weight obtained by the polymerization of the cyclic intramolecular anhydride of not more than two N-carboxya-amino acids in which a carboxyl group and an N-carboxyalnino group are joined to the same carbon atom, said restraining amount being on the order of from 0.002 to 2 grams per liter of emulsion.

11. A photographic silver halide emulsion containing, in a restraining amount, a polypeptide of low molecular Weight obtained by the homopolymerization of the cyclic intramolecular anhydride of a single N-carboxy-a-amino acid in which a carboxyl group and an N-carboxyamino group are joined to the same carbon atom, said restraining amount being on the order of from 0.002 to 2 grams per liter of emulsion.

12. A photographic silver halide emulsion containing, in a restraining amount, a polypeptide of low molecular weight soluble in water miscible alcohols, obtained by the polymerization of the cyclic intramolecular anhydride of not more than two N-carboXy-a-amino acids in which a carboxyl group and a N-carboxyamino group are joined to the same carbon atom, said restraining amount being on the order of from 0.002 to 2 grams per liter of emulsion.

13. A photographic silver halide emulsion containing, in a restraining amount, a polypeptide of low molecular weight soluble in a water miscible alcohol obtained by the homopolymerization of the cyclic intramolecular anhydride of a single N-carb'oxy-a-amino acid in which a carboxyl group and an N-carboxyarnino group are joined to the same carbon atom, said rest-raining amount being on the order of from 0.002 to 2 grams per liter of emulsion.

14. A photographic silver'halide emulsion according to claim 12 which has gelatin as a colloidal carrier for the silver halide.

15. A photographic silver halide emulsion according to claim 12 which has a vinyl alcohol polymer as a colloidal carrier for the silver halide.

16. A photographic emulsion according to claim 12 wherein a mixture of polypeptides as defined in said claim is employed.

References Cited in the file of this patent UNITED STATES PATENTS 1,696,830 Wulff et al Dec. 25, 1928 1,719,711 'Matthi'es et a1. July 2, 1929 1,727,866 Dieterle et al Sept. 10, 1929 2,142,311 Heidenhain Jan. 3, 1939 2,240,471 Swan Apr. 29, 1941 2,397,866 McQueen Apr. 2, 1946 2,454,001 Mueller Nov. 16, 1948 FOREIGN PATENTS 271,039 Great Britain Feb. 25, 1927 272,867 reat Britain May 9, 1927 259,926 Great Britain Sept. 22, 1927 186,864 Switzerland Dec. 16, 1936 OTHER REFERENCES I. A. C. 'S. 69, 1551-2, 1947.

Berichte 39, 857-61, 1906.

Mees: Theory of the Photographic Process, Macmillan, 1942, pp. 63-64.

Sheppard: Gelatin in Photography, vol. I, Eastman Kodak C0,, 1923, page 175. 

1. A PHOTOGRAPHIC MATERIAL COMPRISING A BASE AND A LIGHT SENSITIVE SILVER HALIDE EMULSION CONTAINING, IN A RESTRAINING AMOUNT, A POLYPEPTIDE OBTAINED BY THE POLYMERIZATION OF THE CYCLIC INTRAMOLECULAR ANHYDRIDES OF NOT MORE THAN TWO N-CARBOXY-A-AMINO ACIDS IN WHICH A CARBOXYL GROUP AND AN N-CARBOXY AMINO GROUP ARE JOINED TO THE SAME CARBON ATOM, SAID RESTRAINING AMOUNT BEING ON THE ORDER OF FROM 0.002 TO 2 GRAMS PER LITER OF LIQUID EMULSION USED IN THE PREPARATION OF THE EMULSION LAYER. 